Re: RFC: one approach to fixing PR 14100

[Mark Kettenis <mark.kettenis@xs4all.nl>]

> From: Tom Tromey <tromey@redhat.com>
> Date: Mon, 06 Aug 2012 09:30:59 -0600
> 
> >>>>> "Tom" == Tom Tromey <tromey@redhat.com> writes:
> 
> Tom> Then we need a cleanup instead.
> Tom> I'll look at it on Monday.
> 
> Here it is.
> I regression-tested it on x86-64 Fedora 16.
> 
> Tom
> 
>     	* dwarf2-frame.c (clear_pointer_cleanup): New function.
>     	(dwarf2_frame_cache): Use it.
>     	* frame-unwind.h (frame_sniffer_ftype): Document prologue
>     	cache initialization constraint.

Sorry, but I really think you're working around a problem in the
tailcall sniffer here.  The tailcall sniffer seems to violate several
of the design principles of the frame unwinder framework.  It should
be fixed instead.

> diff --git a/gdb/dwarf2-frame.c b/gdb/dwarf2-frame.c
> index 741a103..986aaea 100644
> --- a/gdb/dwarf2-frame.c
> +++ b/gdb/dwarf2-frame.c
> @@ -994,10 +994,20 @@ struct dwarf2_frame_cache
>    void *tailcall_cache;
>  };
>  
> +/* A cleanup that sets a pointer to NULL.  */
> +
> +static void
> +clear_pointer_cleanup (void *arg)
> +{
> +  void **ptr = arg;
> +
> +  *ptr = NULL;
> +}
> +
>  static struct dwarf2_frame_cache *
>  dwarf2_frame_cache (struct frame_info *this_frame, void **this_cache)
>  {
> -  struct cleanup *old_chain;
> +  struct cleanup *reset_cache_cleanup, *old_chain;
>    struct gdbarch *gdbarch = get_frame_arch (this_frame);
>    const int num_regs = gdbarch_num_regs (gdbarch)
>  		       + gdbarch_num_pseudo_regs (gdbarch);
> @@ -1017,6 +1027,7 @@ dwarf2_frame_cache (struct frame_info *this_frame, void **this_cache)
>    cache = FRAME_OBSTACK_ZALLOC (struct dwarf2_frame_cache);
>    cache->reg = FRAME_OBSTACK_CALLOC (num_regs, struct dwarf2_frame_state_reg);
>    *this_cache = cache;
> +  reset_cache_cleanup = make_cleanup (clear_pointer_cleanup, this_cache);
>  
>    /* Allocate and initialize the frame state.  */
>    fs = XZALLOC (struct dwarf2_frame_state);
> @@ -1111,6 +1122,7 @@ dwarf2_frame_cache (struct frame_info *this_frame, void **this_cache)
>  	{
>  	  cache->unavailable_retaddr = 1;
>  	  do_cleanups (old_chain);
> +	  discard_cleanups (reset_cache_cleanup);
>  	  return cache;
>  	}
>  
> @@ -1226,6 +1238,7 @@ incomplete CFI data; unspecified registers (e.g., %s) at %s"),
>  				 (entry_cfa_sp_offset_p
>  				  ? &entry_cfa_sp_offset : NULL));
>  
> +  discard_cleanups (reset_cache_cleanup);
>    return cache;
>  }
>  
> diff --git a/gdb/frame-unwind.h b/gdb/frame-unwind.h
> index f82d763..aa58640 100644
> --- a/gdb/frame-unwind.h
> +++ b/gdb/frame-unwind.h
> @@ -44,7 +44,9 @@ struct value;
>  
>  /* Given THIS frame, take a whiff of its registers (namely
>     the PC and attributes) and if SELF is the applicable unwinder,
> -   return non-zero.  Possibly also initialize THIS_PROLOGUE_CACHE.  */
> +   return non-zero.  Possibly also initialize THIS_PROLOGUE_CACHE; but
> +   only if returning 1.  Initializing THIS_PROLOGUE_CACHE in other
> +   cases (0 return, or exception) is invalid.  */
>  
>  typedef int (frame_sniffer_ftype) (const struct frame_unwind *self,
>  				   struct frame_info *this_frame,
>